Batteries have started to be used as a power source for transportation equipment, machining equipment, power storage equipment and the like, in addition to be used as a driving power source for electric equipment and electronic equipment. The batteries used for transportation equipment, machining equipment, and power storage equipment are required to provide an instantaneous or continuous high output. A battery group formed by connecting a plurality of battery cells, or a battery module formed by further connecting these battery groups satisfies such a requirement.
For example, Patent Document 1 discloses a battery group comprising a plurality of battery cells connected in parallel, from which electric power is extracted through lead terminals. In this battery group, the connection resistance between a battery cell whose temperature tends to increase at a higher rate and the lead terminals is set larger than that between a battery cell whose temperature tends to increase at a lower rate and the lead terminals. Patent document 1 teaches that in the battery group comprising a plurality of battery cells connected in parallel, variations in capacity among the battery cells can be reduced by suppressing variations in temperature among the battery cells.
Patent Document 2 discloses a secondary battery system including: a secondary battery main body comprising battery modules connected in parallel, the battery modules each being formed by connecting a plurality of unit batteries; and a control unit for controlling the temperature of each battery module so that every battery module has the same internal electrical resistance.
Both of the techniques disclosed in Patent Documents 1 and 2 intend to suppress variations in temperature that might occur among the battery cells or among the battery modules.